This invention relates to a circular polarization antenna and, more particularly, to an orthogonal dual polarization common array antenna of high performance, wide frequency coverage and high discrimination.
In satellite communication with respect to ships, aircrafts, marine buoys, etc. the position and orientation of moving objects change with time with respect to electromagnetic waves arriving from a satellite, so that circular polarization antennas which do not require polarization tracking are used. Also, it is prescribed to use circular polarized waves for direct broadcasting via satellite in the 12-GHz band. Systems adopting the circular polarization require circular polarization antennas, which have excellent polarization characteristics and impedance characteristics over wide band. Further, frequency re-use systems where orthogonal polarization at an identical frequency are used particularly require antennas of high polarization discrimination.
Turnstile antennas have heretofore been most extensively used as circular polarization antennas. In this kind of antennas, two half-wave dipoles are disposed orthogonally and furnished with power in a 90-degree phase shift relationship. In the antenna of this type, if a frequency deviation from the center frequency occurs due to the structure of feed lines and frequency characteristics of a hybrid circuit, the 90-degree phase difference can no longer be maintained to result in elliptical polarization even in the boresight direction. Further, even if the phase difference of 90 degrees is maintained, the circular polarization is deteriorated in the off-axis region due to the difference between the E- and H-plane radiation patterns of the dipole antenna.
An antenna to be fed with equal amplitude and 90-degree phase shift at two points of a rectangular or circular microstrip patch antenna, is based on the same principles as the turnstile antenna noted above. This antenna is thin in shape and light in weight. On the demerit side, however, the frequency coverage of this antenna is generally narrower than that of a dipole antenna. There have been attempts to increase the frequency coverage by using thick substrate of low dielectric constants, e.g., honeycomb substrate. In this case, such problems as disturbance of the radiation pattern due to generation of higher modes and high price of the substrate arise.
It has been proposed an array antenna structure to be described hereinafter in order to solve the various problems in cases where the prior art circular polarization antenna described above is used in a vehicle.
More specifically, in a case where an element antenna does not have sufficiently broad circular polarization characteristics or impedance characteristics, it is thought to construct an array antenna in such a manner as to increase the frequency coverage. As a prior art system based on the technology noted above there is one, in which a pair of elements constitutes a unit structure of an array (Haneishi, Yoshida, Goto, "Patch Antennas and Their Pairs", Papers of Technical Group on Antennas and Propagation, A.P 81-102, November 1981. In this system, two elliptically polarized antennas are disposed in a 90-degree orientation angle difference relationship and excited in 90-degree phase shift relationship. Perfect circular polarization can be obtained in the boresight direction irrespective of the polarization factor of the individual elements of the two-element array. This system can be regarded as a modification of the turnstile antenna noted above. However, a two-element pair array antenna can be constructed only when the elements in the array are even in number, and the system noted cannot be applied to, for instance, circular aperture antennas with triangular arrangement of element. Further, there are limitations on the frequency coverage of the method described.